cDNA clones corresponding to the major keratins expressed in mouse epidermis have been isolated and characterized. Using a combination of in situ hybridization with RNA probes, which are specific for individual keratin mRNAs, and indirect immunofluorescence with monospecific antisera, which were elicited with synthetic peptides corresponding to unique sequences within each keratin subunit, it is possible to show that these keratin genes belong to at least three subsets: those expressed predominantly in the proliferating basal layer of the epidermis, those expressed predominantly in the differentiated suprabasal layers of the epidermis, and those only expressed under hyperproliferative conditions such as in benign and malignant epidermal tumors, hyperplasia induced by the tumor promoter TPA and squamous metaplasia induced by various means in hamster trachea. Genes representing each subset have been isolated and sequenced. Various strategies have been employed to identify sequences regulating the expression of these genes, including vector constructs using different regions of the genomic clones to drive expression of the neomycin-resistance gene and the chloramphenicol acetyl transferase gene and the production of transgenic mice containing genomic fragments encoding human keratins. The in situ hybridization technique has been used to confirm previous results obtained with monospecific antisera which demonstrated that malignant epidermal tumors could be distinguished from benign tumors due to their failure to express the differentiation-associated keratins. The in situ hybridization technique has also been used to study the expression of keratin genes in human biopsies of epidermal tumors and other skin disorders. In addition, this technique has been used to localize transcripts of other genes that are differentially expressed in the epidermis such as genes coding for a thiol proteinase inhibitor, a cornified envelope protein, an activated Harvey ras oncogene and a keratin filament aggregating protein.